The resolution power of a lens is directly proportional to the lens' numerical aperture. Microscope objective lenses having large numerical apertures have been designed and developed in order to improve resolution power. It is usually very difficult to design microscope objective lenses having the same numerical aperture with substantially different lens magnifications.
The above design problem becomes even more problematic when the lens requires an almost constant field of view independent of magnification, as in the case of an immersion microscope objective lens. It is also more difficult to design an anastigmatic low magnification objective lens system having a large numerical aperture. For example, a 40.times. objective with a NA of 1.3 has a larger NA on the image side than a 60.times. which has a NA of 1.4. Therefore, the 40.times. objective is more difficult to design.